1. Field of the Invention
The present invention relates to a vacuum cleaner and more particularly to a vacuum cleaner which enables the user to detect when a filter unit of the cleaner is absent.
2. State of the Art
Modern vacuum cleaners generally operate on a “clean-air fan” principle, according to which the airflow through the cleaner is created by a fan located in the airstream at a point downstream of the separator arrangement which removes dirt and dust from the incoming suction airflow. Dirty air is drawn into the cleaner through a floor-engaging nozzle or other suction inlet and passes through a conduit to the separator arrangement, connected in fluid communication with the inlet of the fan. The airflow thus passes through the separator arrangement and the fan, and is then expelled from the cleaner, to ambient, through an exhaust outlet. The separator arrangement may take any one of several forms, of which filter bags and cyclone separators are the most common, although other arrangements including rigid porous dirt containers are also known. In conventional vacuum cleaner fan systems, the fan and the electric motor which drives the fan are integrated into a compact unit in which the air leaving the fan passes over the motor to effect cooling of the motor. It will be appreciated that should the separator arrangement fail effectively to remove dirt and impurities from the airflow before this airflow reaches the fan, then this dirt could cause damage to the fan and its motor. For this reason it is conventional to provide an additional stage of filtration between the separator arrangement and the inlet to the fan, this additional filtration stage generally being known as the pre-motor filter.
Since pre-motor filters must be removed from the cleaner periodically for cleaning or renewal, it will be appreciated that situations can arise where the user operates the cleaner with the pre-motor filter absent. This is clearly undesirable, and a number of arrangements have been proposed hitherto for preventing the occurrence of such situations. For example, European patent application EP 0,895,744 discloses an arrangement in which the presence of the pre-motor filter is detected by an electronic scanning system: an electronic sensing device is mounted on the body of the vacuum cleaner and co-operates with a sensing element attached to the filter to produce a signal dependent on the presence or absence of the filter, or indeed on the presence of the correct type of filter and/or its correct installation in the cleaner; the signal may be used to initiate a filter-absent warning to the user and/or to prevent the fan motor from starting. This filter-presence detection system is complicated and is expensive to manufacture, rendering it unsuitable for simple low-cost products.
A number of other detection systems have been proposed, for detecting the presence or absence of the pre-motor filter, based on the principle of preventing the use of the cleaner unless the cover which closes the separation chamber is correctly installed. A typical arrangement of this type is disclosed in European patent application EP 1,214,903, in which correct installation of the filter results in a spring-loaded tongue being withdrawn from a position in which it would obstruct closure of the dirt receptacle: without the dirt receptacle being properly closed, the cleaner cannot function. This and other mechanical arrangements are generally complicated and increase the manufacturing costs unduly.